this is all it took to get me running. There were further tweaks, but no errors about failing to execute login command.

only real difference between mine and audiodef's bit above, he's using an .xinitrc found in a given user's home directory. I'm using that environment variable. They accomplish the same thing different ways. Well, mostly the same.

If you want every user to default to fluxbox, the /etc/env.d/90xsession route is the one to go
if you want to set window manager or desktop environment on a per user basis, the thing to do is create an .xinitrc in the home directory of every use r (for those desiring fluxbox, just contains exec startfluxbox), and put in the requisite startup commands for the desired desktop, and skip setting the env variable.